Added reminder functionality

Caveat: this is the first time I have contributed to a github project, so if I've made a mistake in the procedure, please let me know (the first thing I did was to submit the patch via. email... so insanum can disregard that as I'm trying to do it the 'right way').

I needed better functionality for specifying reminders, so I've made the following modifications:

1) You can now enter multiple reminders.  The reminders may be entered as multiple --reminder options or, if being prompted by gcalcli, you can enter any number until you enter a '.' to end input.

2) Each reminder can be a number (the number of minutes as it has always been), or it can be a string in one of the forms:

        'LEN'
        'LEN METH'

Here LEN is a number followed by a single character 'w', 'd', 'h', or 'm' which says that the length is entered in weeks, days, hours, or minutes.

METH is a string 'popup', 'email', or 'sms' so you can explicitly set the notification method for each reminder.  If no method is specified, it defaults to 'popup' so it is completely compatible with the current behavior.

Example:
     '3d email'    : send email 3 days in advance

3) Previously, you HAD to enter a reminder, but I wanted to create events that showed up but did not have reminders.  Now, you can enter NO reminders, in which case the default reminders for the calendar are used, but you can also enter the new flag --nodefault_reminders to create an event without any reminders at all.